Liquids to be clarified by the method of the invention are generally waters occuring at the outlet from an industrial process and intended, for example, either for further industrial use or else for rejection into the public waste system or the ground. Although the following description refers to practical examples in which the method is implemented with waste water from the minerals or papermaking industries, the method may naturally be applied to clarifying a wide variety of charged liquids.
Heretofore, the methods used have required relatively long periods of time to separate and sediment out solid particles in suspension so as to obtain a liquid or water which is sufficiently clarified to meet the standards laid down. This arises because the vertical settling speed does not exceed a few meters per hour, and necessitates the use separator equipment which is expensive, and of large size and volume.
It is widely known that in order to increase the rate at which solid particles settle it is necessary to link up the particles in suspension. To this end, a small quantity of additive is added to the charged liquid in order to initiate the flocculation phenomenon. This forms agglomerates or flakes of considerably greater size than the particles from which they are built up.
Generally, after flocculation, the liquid and its charge of flakes of different sizes is injected at an intermediate level into the sole compartment of a clarification and settling apparatus of the "single stage" type. This compartment contains flakes at a higher concentration. This makes it possible to accelerate the increase in flake size and consequently to increase the settling speed. The flakes which have settled in the lower portion of the settling zone in this compartment are concentrated and they are removed (generally continuously, but sometimes in a discontinuous manner) in the form of a sludge which is more or less concentrated, and with a suitably determined average flow rate. The clarified liquid is removed at the same time from the top of the settling device from which it merely overflows.
It is also known that the two functions of clarifying the liquid and concentrating the solid can be obtained more rapidly by separating the functions so that they take place in distinct parts of the apparatus. The apparatus is then said to have two stages. The stages may be superposed with the clarification stage being above the concentration stage, or they may be concentric, or they may be side-by-side.
It is known that the flocculation phenomenon as initiated by flocculation additives forms agglomerates whose mechanical cohesion is low, and that hydraulic movements in the clarification zone may have both advantageous and disadvantageous effects simultaneously on the structure and the physical characteristics of the agglomerates.
Speeds which are too high in some portions of the apparatus break up prior formed flakes: the finally formed flakes have too wide a range of sizes including many very fine agglomerates which settle poorly. Speeds which are too slow in other portions of the apparatus enable flakes to form which are too big and which agglomerate with one another. These flakes then form continuous layers of low permeability which slow down the passage of the liquid to be clarified. Instability phenomena then occur with some zones being formed in which the liquid moves at high speeds that destroy the flakes.
More particularly, a first known clarification method applicable to a liquid charged with fine particles comprises the following operations:
adding a flocculation additive to a charged liquid to cause those of said fine particles which meet one another to bind together and form flakes;
substantially continuously inserting said mixture into a distribution chamber having substantially uniform pressure and inserting said mixture into a clarification chamber via a plurality of injectors distributed over the surface of said chamber, at the bottom thereof;
removing clarified liquid via a liquid-removal orifice in the upper portion of said chamber; and
removing sludge via a sludge-removal orifice situated further down.
The charged liquid injection rate is selected in such a manner as to form a belt of sludge above the injectors with the charged liquid rising through said belt between flakes which have already been formed, and the fine particles still in suspension in said water binding to said flakes in order to increase the volume thereof so that the liquid arrives clarified above said belt of sludge and so that a layer of clarified liquid forms above the belt of sludge.
Each of said injectors is provided with guide means.
This known method is described in French Pat. No. 2 477 896 (Ghezail et al.). More particularly, it includes the following characteristics:
The injectors are injection orifices through the floor of the clarification chamber, and the said guide means are funnel-shaped elements flaring upwardly from said orifices. In each horizontal plane passing through such a funnel, the cross-section of the passage allowed to the injected charged water is, according to said patent (page 5, lines 1-5), constituted by the entire interior of the funnel, thereby very rapidly reducing the speed of said water after it leaves the injection orifices. This is the desired result, since in this method it is desired above all that the belt of sludge above the injection orifices should be substantially free from agitation. In practice, once the liquid passes through the injection orifice at any substantial speed, an axial cylindrical jet is formed at a distance from the walls of the funnel. Further, in this method, the sludge is partially removed on a temporary basis in a periodic manner through the injection orifices by applying temporary suction thereto and causing the flow direction through the injection orifices to be temporarily and suddenly reversed. The walls of the funnels then appear to constitute means for guiding the sludge towards said orifices. These brief periodic reversals of charged water flow direction prevent, to some extent, the formation of a continuous and insufficiently permeable layer which would cause the above-mentioned instability phenomena to appear.
A second known method makes it possible to avoid, to some extent, the formation of a continuous and insufficiently permeable layer of sludge. This method is described in French Pat. No. 1 115 038 (Degremont) and makes use of periodic temporary accelerations in the flow of charged water through the injection orifices.
A third method is known from French Pat. No. 2 431 317 (Dorr Oliver) corresponding to U.S. Pat. No. 4,263,137 (Kos). This method uses moving injectors including funnel-shaped guide means which serve to slow down and to distribute the flow of injected charged water.
A fourth known method is described in German patent specification 2,238,959 (H. Scheven).
In that method, the mixture of charged liquid and flocculation agent is inserted into a clarification chamber via a plurality of injectors distributed over the area of the chamber with each of the injectors forming a downwardly directed vertical jet towards a cup-shaped deflector. The deflector divides the jet and spreads it radially and upwardly, i.e. towards a sludge belt. It is specified that the association of said cups with said injectors creates "a slow uniform upward current". This method does not appear to have been developed industrially.
The object of the present invention, during the implementation of a clarification method of the above-mentioned type, is to increase, in a simple manner, the average speed at which the rising liquid passes through the sludge belt and/or to treat charged liquid containing a higher concentration of solid material, while still retaining efficient separation with the same type of solid particles, and using the same type of flocculation additive at the same concentration relative to the mass of solid material.
Any increase in said average rising speed gives rise to an increased flow rate of treated charged liquid through an installation of given area, or alternatively, for a given flow rate of liquid to be treated, it gives rise to a reduction in the area required by the installation. When a high flow rate of liquid is to be treated, such a reduction in the area of the installation constitutes an important economic advantage.